1. Flammability Limits
As explained in Lees, F. P., "Loss Prevention in the Process Industries, Volume 1," 485-86 (1980) and Coffee, R. D., Loss Prevention 13, 74-80, (1980), a flammable gas, e.g., methane, butane, ethylene, butadiene, and other hydrocarbons, burns in oxidizing environments only over a limited composition range. The limits of flammability (often called the explosive or hot flame limits) are the concentration extremes at which a mixture of a flammable gas and an oxidant can continue to burn once a flame is ignited by an external energy source such as a spark. These flammability extremes are a function of temperature, pressure, and composition. The explosive limit is usually expressed as volume or mole percent of flammable gas in a mixture of oxidant (usually oxygen), inert, and flammable gas. The smaller value is the lower (lean) limit and the larger value is the upper (rich) limit. For example, methane-oxygen mixtures will propagate flames for methane concentrations between 5.1 and 61 mole percent methane (i.e., 94.9 and 39 mole percent oxygen) and methane-air mixtures between 5.3 and 14 mole percent methane (i.e., 19.9 and 18 mole percent oxygen), at 25.degree. C. and atmospheric pressure. In general, the lower explosive limit (LEL) decreases, and the upper explosive limit (UEL) increases as temperature and pressure increase, and amount of inert decreases. Below a certain oxygen content, called the minimum oxygen content (MOC), the mixture will not support combustion. For methane at 25.degree. C. and 1 atmosphere, the MOC is 13.98 mole percent oxygen.
2. Diluents for Gas-Phase Epoxidation Reactions
When carrying out a highly exothermic reaction (e.g., epoxidation of butadiene or ethylene), it is important to design the reactor for adequate heat removal to prevent thermal runaway (uncontrollable reaction and generation of heat). A typical reactor configuration for operation of a highly exothermic reaction is a multi-tubular packed bed immersed in a flowing heat transfer fluid (often boiling water). In such a reactor, heat is removed via several mechanisms: (1) axial convection from the catalyst surface to the bulk gas feedstream; (2) radial convection through the catalyst and support particles to the tube walls and into the heat transfer fluid; and (3) radial conduction from the catalyst surface through the bulk gas to the tube walls and into the heat transfer fluid. If the tubes are too large in diameter (radial temperature gradient is large), the reactant or diluent gas has low heat capacity, or the gas flow is too low, hot spots will develop which can lead to a runaway reaction or catalyst deactivation.
Processes for the selective epoxidation of olefins which contain no allylic hydrogen atoms (non-allylic olefins) or olefins which contain hindered allylic hydrogen atoms are described by Monnier and Muehlbauer in U.S. Pat. Nos. 4,897,498; 4,950,773; 5,081,096; 5,138,077; and 5,145,968. Stavinoha and Tolleson disclose in U.S. Pat. No. 5,117,012 the selective epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene (EpB) by contacting a mixture comprising 1,3-butadiene, oxygen, and methane with a supported silver catalyst at elevated temperatures. Stavinoha et al. disclose in U.S. Pat. No. 5,362,890 an improved process for the selective epoxidation of 1,3-butadiene to EpB wherein the ballast gas for the reaction is n-butane. Boeck et al. disclose in U.S. Pat. No. 5,618,954 a similar process for the epoxidation of 1,3-butadiene to EpB with nitrogen or C.sub.1 -C.sub.4 hydrocarbons as the diluent.
The use of diluent gases in non-allylic olefin epoxidation, specifically the epoxidation of 1,3-butadiene to 3,4-epoxy-1-butene, is described in U.S. Pat. Nos. 5,362,890, and 5,618,954. U.S. Pat. No. 5,618,954 discloses that nitrogen and C.sub.1 -C.sub.4 paraffinic hydrocarbons, especially methane, or mixtures thereof are the preferred diluents for the epoxidation of 1,3-butadiene to EpB. The oxygen:butadiene ratio in the reactor feed gas can be increased by using methane as the diluent over that with nitrogen as the diluent without the methane:oxygen:butadiene mixture becoming flammable.
U.S. Pat. No. 5,362,890 discloses the use of C.sub.2 -C.sub.6 paraffin hydrocarbons as diluents for non-allylic olefin epoxidation. The data disclosed in this patent shows the advantages of using higher alkane hydrocarbons over methane, nitrogen, and other common diluents. The advantages cited include higher safe oxygen levels, higher epoxide production levels for the same reactor temperatures, and more stable operation due to better heat removal.
The use of diluent or ballast gases in ethylene epoxidation is described in Canadian Patent Nos. 1,286,687 and 2,053,404; and U.S. Pat. Nos. 3,119,837 and 5,057,481. According to these patents, the typical volumetric composition of feed gases used in such ethylene epoxidation processes comprise 5 to 50 volume percent ethylene, 2 to 8 volume percent oxygen, up to 7 volume percent carbon dioxide, up to 5 mole percent ethane with the balance being composed of an additional inert diluent such as nitrogen or methane.
U.S. Pat. No. 3,119, 837 discloses that selectivity of ethylene conversion to ethylene oxide can be enhanced by the addition of methane as a diluent. Methane serves as a heat sink, moderating temperature differentials within the reactor, and allows for more isothermal reactor operation. This patent further states that the benefits to selectivity and ease of operation do not extend to other paraffins normally encountered in commercially available ethylene, e.g., ethane and propane, due to excessive stripping of chlorine from the surface of the catalyst, which renders the catalyst unstable and, thus, susceptible to thermal runaway. Use of methane is also said to allow an increase in the oxygen:ethylene ratio in the reactor feed gas over the ratio with nitrogen, which increases conversion of ethylene to ethylene oxide.
According to Canadian Patent 1,286,687, other diluents that function as heat sinks include nitrogen, helium, argon, carbon dioxide, and lower paraffins such as methane and ethane. However, U.S. Pat. No. 5,057,481 discloses that the use of ethane at concentrations greater than about 5 mole percent results in reduced selectivity in the epoxidation of ethylene to ethylene oxide and lower thermal stability because the chloride concentration on the catalyst surface is lowered. Typical silver catalysts employed in the epoxidation of ethylene contain about 1 to 300 parts by million by weight (ppmw) of Cl on the catalyst surface, both to increase selectivity to ethylene oxide by lowering combustion of ethylene and ethylene oxide to carbon dioxide and water as well as to increase the thermal stability of the silver catalyst. If the level of Cl on the surface of the silver catalyst becomes too low, the reaction becomes excessively exothermic with accompanying loss of selectivity. Ethane acts as a chloride stripping agent and at concentrations above 5 mole percent and at temperatures typically employed in the epoxidation of ethylene, e.g., 230 to 280.degree. C., the degree of chloride stripping becomes unacceptably excessive. As is disclosed in the above-cited patents, one of the problems associated with the use of carbon dioxide as a heat transfer agent (heat sink) in ethylene epoxidation processes is that at levels greater than about 7 mole percent carbon dioxide becomes a reaction inhibitor for ethylene oxide formation. Thus, the concentration of carbon dioxide in feed gas in ethylene epoxidation processes must be limited to concentrations of less than about 7 mole percent. At such low levels, carbon dioxide does not have an appreciable effect on the heat capacity nor the flammability characteristics of the reaction gas mixture.
Although much of the art discussed above has resulted in improvements in the efficiency, activity, and/or stability of the epoxidation catalyst, there still exists a need in the art to further improve and increase the efficiency, activity, and stability of such catalysts. Accordingly, one of the objects of the present invention is to provide a process that meets this need in the art.